Multi-mode TDMA spread spectrum communication system

ABSTRACT

A simple and flexible over-air protocol for use with a mobile telephone system, having hand-held telephones in a microcell or other type of cellular communication system. A method in which user stations communicate with one or more base stations to place and receive telephone calls, in which the user stations are provided a secure voice or data link and have the ability to handoff calls between base stations while such calls are in progress. Each base station has a set of &#34;air channels&#34; to which it transmits in sequence. The air channels supported by each base station are called that base station&#39;s &#34;polling loop&#34;. A user station receives general polling information on an unoccupied air channel, transmits responsive information to the base station, and awaits acknowledgment from the base station. Each base station may therefore simultaneously maintain communication with as many user stations as there are air channels in its polling loop. The ability of a user station to communicate on any unoccupied air channel makes the protocol air-channel agile, while the stability of user station and base station clocks may define air channels, gaps, and minor frames.

RELATED APPLICATION DATA

"This is a continuation of application Ser. No. 08/476,936, filed Jun.7, 1995, now abandoned, which is a continuation of copending applicationSer. No. 08/284,053, filed on Aug. 1, 1994" which is acontinuation-in-part of application Ser. No. 08/215,306, filed Mar. 21,1994, now abandoned, which is a continuation-in-part of application Ser.No. 08/146,496, filed Nov. 1, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of communications, and particularlyto communication systems using spread spectrum techniques and toover-the-air protocols for mobile telephones.

2. Description of Related Art

A mobile telephone system may generally comprise a set of "userstations", typically mobile and the endpoints of a communication path,and a set of "base stations", typically stationary and theintermediaries by which a communication path may be established ormaintained. In a mobile telephone system, one important concern is theability of mobile stations to communicate with base stations in asimple, flexible and rapid manner. The communication protocol betweenuser stations and base stations should be rapid, so that user stationsare not required to wait to establish a communication path. The protocolshould be simple, so that user stations need not incorporate expensiveequipment to implement it. The protocol should be flexible, so that userstations may establish communication paths in as many communicationenvironments as reasonably possible.

Accordingly, it would be advantageous to provide a simple and flexibleover-air protocol for use with a mobile telephone system. One class ofsystems in which this would be particularly advantageous is that ofpersonal communication systems, particularly those with hand-heldtelephones in a microcell or other type of cellular communicationsystem.

SUMMARY OF THE INVENTION

The invention provides in one aspect a simple and flexible over-airprotocol for use with a mobile telephone system, such as a PersonalCommunication System (PCS) with handheld telephones in a cellularcommunication system. A preferred embodiment is adapted to "pocketphones", i.e., small hand-held telephones which may use a cellularcommunication technique, but the invention may be used with any cellularor mobile telephone system. The protocol defines a method in which userstations, such as cellular or mobile telephone handsets, communicatewith one or more base stations to place and receive telephone calls. Theprotocol provides air-channel agility between base stations and userstations, while providing a secure voice or data link and the ability tohandoff calls between base stations while they are in progress.

In a preferred embodiment, each base station may have a set of "airchannels" which it polls, e.g. by transmitting to each one in sequence.The air channels supported by each base station are referred to as a"polling loop" for a particular base station. A user station may receiveinformation on an unoccupied air channel, receive the base station'stransmission, and transmit information to the base station. Each basestation may therefore simultaneously maintain communication with as manyuser stations as there are air channels in its polling loop. The abilityof a user station to communicate on any unoccupied air channel makes theprotocol air-channel agile. Each base station continually transmits oneach one of its air channels in a predetermined sequence. Each basestation transmission may be followed by a first gap, a user stationtransmission (if some user station attempts to communicate), and asecond gap, before the base station transmits on the next air channel. Abase station transmission, first gap, user station transmission, andsecond gap are collectively called a "minor frame". A polling loop inwhich each air channel is polled is called a "major frame".

In a preferred embodiment, stability of user station and base stationclocks may define the air channels, gaps, and minor frames. The userstation may synchronize itself to the base station's clock by detectinga minor frame and by adjusting its clock to be in synchrony with thebase station when the first bit sequence of the minor frame is detected.The stability of the user station and base station clocks may then holdthe user station and base station in synchronization, as long as theuser station is periodically able to receive transmissions from the basestation. Should reception in either direction be interrupted for toolong, the base station and user station clocks may drift apart and theuser station may need to reacquire the transmission from the basestation.

Handoffs are preferably initiated from the user station whichcontinually monitors available air channels from the same and competingbase stations during dead time. A user station may handoff within thesame polling loop to establish communication in a new minor frame, ormay handoff in such a manner to establish communication in a new minorframe within a polling loop of a different base station. In the lattercase, a base station controller may assist in transferring the call fromone base station to another.

The invention provides in yet another aspect for closed loop powercontrol in the user stations by monitoring and adjusting the userstation power at regular intervals, such as once in each major frame.The control of user station power serves to reduce intercellinterference and prolong battery life in mobile handsets.

Variable data rates are provided in another aspect of the presentinvention. A user station may increase its data rate by transmittingand/or receiving in multiple minor frames during a major frame, or mayreduce its data rate by transmitting and/or receiving in fewer thanevery major frame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram of a communication system having base stations anduser stations.

FIG. 1B is a diagram of a preferred cellular environment in which theinvention may operate.

FIG. 1C is a diagram of a network architecture showing various systemcomponents.

FIG. 2 is a diagram of frame and message formats in a polling loop.

FIG. 3 is a diagram showing formats for message types.

FIG. 4 is a diagram of a network architecture showing connectionsbetween base stations and a network.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The disclosure of the invention may be supplemented by the contents oftechnical information appended to this specification in a TechnicalAppendix A, a Technical Appendix B, and a Technical Appendix C, each ofwhich is hereby incorporated by reference as if fully set forth herein.No admission is made as to possible prior art effect of any part of theappendix.

In a preferred embodiment, it is contemplated that communication betweenbase stations and user stations will be conducted using aspread-spectrum technique. There are at least three methods forestablishing synchronization and communication, each preferably using anM-ary technique in which multiple bits of data are transmitted for eachspread-spectrum symbol, e.g., by transmitting and receiving multipledifferent spreading codes, and interpreting the received one of thosemultiple different spreading codes at the receiver to indicate multipledata bits. Synchronization may be accomplished either by (1) automaticsynchronization disclosed in co-pending application Ser. No. 08/146/491,entitled "DESPREADING/DEMODULATING DIRECT SEQUENCE SPREAD SPECTRUMSIGNALS", Lyon & Lyon Docket No. 200/154, filed on Nov. 1, 1993 in thename of inventors Robert Gold and Robert C. Dixon now abandoned, herebyincorporated by reference, by (2) synchronizing with matched filters, by(3) demodulation and despreading using sliding correlators, or by (4) acombination of these techniques, e.g., matched filters forsynchronization plus sliding correlators for demodulation anddespreading, or matched filters for synchronization plusautosynchronization for demodulation and despreading.

FIG. 1A is a diagram of a communication system having base stations anduser stations.

A communication system 101 for communication among a plurality of userstations 102 may include a plurality of cells 103, each with a basestation 104, typically located at the center of the cell 103. Eachstation (both the base stations 104 and the user stations 102) maygenerally comprise a receiver and a transmitter. The user stations 102and base stations 104 preferably communicate using time divisionmultiple access (TDMA) or time division duplex (TDD) techniques asfurther described herein, in which specified time segments or majorframes are divided into assigned time slots or minor frames forindividual communication.

FIG. 1B is a diagram of a preferred cellular environment in which theinvention may operate. A geographical region is divided into a pluralityof cells 103. Associated with each cell 103 is an assigned frequency andan assigned spread spectrum code. Preferably, three differentfrequencies F1, F2 and F3 are assigned in such a manner that no twoadjacent cells have the same assigned frequency F1, F2 or F3. The effectof such a frequency reuse pattern is to minimize interference betweenadjacent cells.

To further reduce the possibility of intercell interference, differentorthogonal spread spectrum codes C1 through C6 are assigned as shown inadjacent clusters 110. Although six spread spectrum codes C1 through C6are shown in FIG. 1B, it is contemplated that fewer or more spreadspectrum codes may be suitable depending upon the particularinformation. Further information regarding a preferred cellularenvironment may be found in U.S. application Ser. No. 07/682,050entitled "Three Cell Wireless Communication System" filed on Apr. 8,1991 in the name of Robert C. Dixon now U.S. Pat. No. 5,402,413, andhereby incorporated by reference as if fully set forth herein.

The use of spread spectrum for carrier modulation permits a veryefficient frequency reuse factor of N=3 for allocating different carrierfrequencies F1, F2 and F3 to adjacent cells 103. Interference betweencells 103 using the same carrier frequency F1, F2 or F3 is reduced bythe propagation loss due to the distance separating the cells 103 (notwo cells 103 using the same frequency F1, F2 or F3 are less than twocells 103 in distance away from one another), and also by the spreadspectrum processing gain of cells 103 using the same carrier frequenciesF1, F2 or F3.

The preferred spread spectrum bandwidth may differ according to thefrequency band of operation. When operating in the PCS A, B, or Cfrequency bands, each of which is 15 MHz wide, the center frequenciesF1, F2 and F3 are preferably located at 2.5 MHz, 7.5 MHz, and 12.5 MHz,respectively, from the lowest band edge of the A, B or C frequency band.

The PCS D, E, or F bands, on the other hand, are each 5 MHz wide, whichis the same bandwidth as a preferred spreading bandwidth for a spreadspectrum signal used in the particular cellular environment.Consequently, a single carrier frequency is placed in the center of theD, E or F band, and a frequency reuse factor of N=1 is used because thespread spectrum signal covers the entire available bandwidth. Because anN=1 frequency reuse pattern is used, the required intercell interferencerejection must be obtained by spread spectrum code orthogonality and/orthe use of sectorized antenna patterns. The exchange of interfering airchannels or time slots, as described elsewhere herein, may also be usedto mitigate intercell interference.

When operating in the PCS unlicensed band, which has a bandwidth of 20MHz divided into individual channel only 1.25 MHz wide, the spreadspectrum chipping rate may be reduced to approximately 1.25 Mcps. TheTDMA burst rate, or number of TDMA time slots (or minor frames) in eachpolling loop, may also be reduced to maintain the required spreadspectrum processing gain for rejecting intercell interference. Anon-spread spectrum TDMA/TDD signal modulation format for operation inthe unlicensed band may also be provided.

FIG. 1C is a diagram of a network architecture showing various systemcomponents.

A preferred communication system is designed around an object-basedsoftware architecture which allows for flexibility in interconnection tovarious networks including public switched telephone networks, AIN, GSMand IS-41 network infrastructures. It is also contemplated that thecommunication system may interface with a cable television distributionnetwork; however, such an interface may require the addition to thecable television network of a switch architecture, two-way amplifiers,redundancy, and, in order to use the coaxial portion of the cable TVnetwork, a remote antenna subsystem to extend coverage from a basestation 104.

The overall system thus provides flexibility to interface with a varietyof different networks depending upon the desired application. To allowinterconnection to diverse networks, the system uses internalcommunications based on ISDN messages, called "notes", for passingnecessary information among components within the system. These "notes"are so named as not to confuse them with the ISDN specific protocolitself. Network messages (based on, e.g., Q.921, Q.931 protocols, orothers) are converted by the system into "notes" for efficient operationwithin the hardware platform.

In FIG. 1C is shown various components of a preferred systemarchitecture including a plurality of base stations 104 forcommunicating with user stations 102. Each base station 104 may becoupled to a base station controller 105 by any of a variety of linkingmeans 109 including, for example, local area data access (LADA) lines,Tl or fractional Ti lines, ISDN BRI's, cable TV lines, fiber opticcable, digital radio, microwave links, or private lines. As anillustration shown in FIG. 1C, a plurality of base stations 104 may becoupled to base station controller 105 by first connecting to a coaxialcable 111 which is thereafter coupled to a fiber optic cable 113 at afiber node 112. The fiber optic cable 113 is coupled to the base stationcontroller 105 as shown.

Each base station controller 105 may be connected to a network 106 suchas a public switched telephone network (PSTN) or a personalcommunications system switching center (PCSC) by a variety of networklinks 108, which include the same basic categories of transport means asthe linking means 109. Base station controllers 105 may also connect tothe network 106 via an X.25 link 114.

The system of FIG. 1C also incorporates the use of "intelligent" basestation (IBS) 107 compatible with LEC-based AIN architecture that may beconnected directly to a network 106 without the interface of a basestation controller 105. The intelligent base stations 107 may thereforebypass the base station controllers 105 for local handoffs andswitching, and instead perform these functions via the network 106. InAIN based architectures, signaling between network elements may becarried out using standard signaling protocols including, for example,SS7 and IS-41.

In operation, the base stations 104 format and send digital informationto the base station controller 105 (or directly to the network 106 inthe case of an intelligent base station 107). The base stationcontrollers 105 concentrate inputs from multiple base stations 104,assist handoffs between base stations 104, and convert and formatchannel information and signaling information for delivery to thenetwork 106. The base station controllers 105 may also manage a localcache VLR database, and may support basic operations, administration andmanagement functions such as billing, monitoring and testing. Each basestation controller 105, under control of the network 106, may managelocal registration and verification of its associated base stations 104and may provide updates to the network 106 regarding the status of thebase stations 104.

The network 106 connects to the base station controllers 105 for calldelivery and outgoing calls. The connection between the network 106 anda base station controller 105 may utilize the Bellcore "Generic C"interface which includes Q.921, Q.931 and modifications to Q.931.

Intelligent base stations 107 may use ISDN messaging for registration,call delivery and handoff over a public telephone switch. Theintelligent base station 107 may have all the general capabilities of abase station 104 but further incorporate a BRI card, additionalintelligence and local vocoding. The connection between the network 106and an intelligent base station 107 may utilize the Bellcore "Generic C"interface which includes Q.921, Q.931 and modifications to Q.931.

If the network 106 is a GSM network, then base stations 104 may connectto the network 106 through a defined "A" interface. Features andfunctionality of GSM are passed to and from the base stations 104 overthe "A" interface in a manner that is transparent to the end user.

As noted, the system may also interconnect to cable televisiondistribution networks. The base stations 104 may be miniaturized to thepoint where they can be installed inside standard cable TV amplifierboxes. Interfacing may be carried out using analog remote antennasystems and digital transport mechanisms. For example, T1 and FT1digital multiplexer outputs from the cable TV network may be used forinterfacing, and basic rate (BRI) ISDN links to transport digitalchannels.

Cell site diagnostics may be performed remotely through either thecontrol channel on the digital link resident in the base station 104 ora dial up modem for some implementations. Such diagnostics may beperformed on each component board of the base station 104. In addition,the base stations 104 and base station controllers 105 may be remotelymonitored and downloaded with updated software as required. Similarly,user stations 102 can also be downloaded with software over air channelsas required for maintenance purposes or for system upgrades.

The user stations 102 comprise in one embodiment mobile handsets capableof multi-band and/or multi-mode operation. The user stations 102 may bemulti-mode in that they may be capable of either spread spectrumcommunication or conventional narrowband communication. The userstations 102 may be multi-band in the sense that they may be set tooperate on a plurality of different frequencies, such as frequencies ineither the licensed or unlicensed frequency bands.

For example, a user station 102 may be set to operate on any frequencybetween 1850 and 1990 MHz in 625 kHz steps. Thus, each user station 102may have a frequency synthesizers which can be programmed to receive andtransmit on any one of 223 frequencies. If the user station 102 operatessolely in the licensed PCS band, however, the programmable frequencysteps may be in 5 MHz increments, in which case the first channel may becentered at 1852.5 MHz, the next at 1857.5 MHz, and so on. If operatingin the isochronous band between 1920 and 1930 MHz, the first channel maybe centered at 1920.625 MHz, and the channel spacing may be 1.25 MHzacross the remainder of the isochronous band. The user stations 102 neednot operate in the 1910 to 1920 MHz band, which is reserved forasynchronous unlicensed devices.

Further detail regarding the multi-band and multi-mode aspects of userstations 102 may be found in copending U.S. application Ser. No.08/146,492 filed on Nov. 1, 1993 in the name of inventors Robert C.Dixon and Jeffrey S. Vanderpool, entitled "DUAL-MODE WIRELESS UNIT WITHTWO SPREAD-SPECTRUM FREQUENCY BANDS," copending application Ser. No.08/059,021 filed May 4, 1993 in the name of inventors Douglas G. Smith,Robert C. Dixon and Jeffrey S. Vanderpool, entitled "DUAL-BANDSPREAD-SPECTRUM COMMUNICATION," and copending application Ser. No.08/206,045 filed on Mar. 1, 1994 in the name of inventors Robert C.Dixon and Jeffrey S. Vanderpool, entitled "DUAL-MODE TRANSMITTER ANDRECEIVER," each of which is hereby incorporated by reference as if fullyset forth herein. The multi-band, multi-mode capability enables the userstations 102 take advantage of variety of diverse system architecturesas described herein, and to interface with various different networkswith a minimum of hardware or software adjustments.

Base stations 104, like user stations 102, may also be provided withmulti-band and multi-mode capabilities as described above.

Frame And Message Formats

FIG. 2 shows frame and message formats in a polling loop.

In a single cell 103, a base station 104 may poll user stations 102 inthe cell 103. The base station 104 may repeatedly transmit a major frame201, comprising a sequence of minor frames 202. As noted herein, eachminor frame 202 may comprise a polling exchange for a single userstation 102, while each major frame 201 may comprise a complete pollingsweep of user stations 102 in the cell 103.

In a preferred embodiment, the base station 104 may conduct its pollingexchanges using a set of air channels 203. Each of the air channels 203may comprise a separate transmission channel, such as a separatefrequency band for FM or AM encoding, a separate spreading code forspread-spectrum encoding, a separate spatial location, or other divisionof communication slots between base stations 104 and user stations 102.In a preferred embodiment, the base station 104 may poll every one ofits air channels 203 in a predetermined sequence in a single major frame201.

While in a preferred embodiment, the base station 104 may poll every oneof its air channels 203 in a single major frame 201, but it will beclear to those of ordinary skill in the art, after perusal of thisapplication, that the base station 104 may restrict its poll to only aportion of its air channels 203 in each major frame 201, so long as allair channels 203 are eventually polled, and in an order so that eachuser station 102 may determine in which minor frame 202 it shouldrespond.

Each minor frame 202 may comprise a base transmission 204 by the basestation 104, a first gap 205, a user transmission 206 by a user station102 (if any user station 102 responds), and a second gap 207. During thebase transmission 204, a user station 102 desiring to establish acommunication path may receive the base transmission 204 and determineif the air channel 203 is occupied or not. If not occupied, the userstation 102 may respond with its user transmission 206.

In one embodiment, in order to provide efficient service in low densityrural areas, cell radii can be extended to large distances (e.g., beyond8 miles) by providing the increased guard times as would be required forthe longer round trip propagation delays encountered in the largercells. Cells with large radii can be supported by reducing the number ofminor frames 202 per major frame 201 to a lesser number (e.g., from 32to 25). Since such large cell radii will ordinarily be deployed in lowpopulation density areas, reduced cell capacity caused by the smallernumber of minor frames 202 per major frame 201 is not a severe drawback.

In a preferred embodiment, a base transmission 204 may comprise a headerfield 207, which may be a fixed length of sixteen bits, a D field 208,which may be a fixed length of eight bits, and a B field 209, which maybe a fixed length of 160 bits, or may be a variable length. In anembodiment using a variablelength B field 209, the variable length maybe determined in response to the polling loop time and the data ratewhich must be supported. For example, in a preferred embodiment of a30-channel system, the B field 209 may be 160 bits long.

In a preferred embodiment, the user transmission 206 may comprise likefields as the base transmission 204.

The header field 207 may comprise an origin bit 210, which may be a "1"bit for base transmissions 204 and may be a "0" bit for usertransmissions 206. Other parts of the header field 207 may indicateinformation about the base transmission 204 or user transmission 206itself, e.g., what type of message the base transmission 204 or usertransmission 206 comprises. The header field 207 may also comprise a CSCor CRC code 211 (a cyclic redundancy check) having four bits.

The D field 208 may comprise control information to be communicatedbetween base stations 104 and user stations 102 once a communicationlink is established. This control information may generally be used forISDN communication between base stations 104 and user stations 102, suchas control information generally communicated using the ISDN "Dchannel". Because the D field 208 is separate from but simultaneous withthe B field 209 which normally handles the bulk of information transferdue to its higher data rate, the D field 208 may be used for pagingapplications, notifications (e.g., voice mail), short message service(similar to GSM), or other user applications. Thus, the simultaneousnature of the D field 208 and the B field 209 allows messaging functionseven when the user station 102 is "in use".

During link expansion, described with regard to FIG. 3 herein, the Dfield 208 may also comprise a user nickname 212 for communication fromthe base station 104 and a designated user station 102. The usernickname 212 may comprise a temporary identifier for the user station102 selected by the base station 104.

The B field 209 may comprise data, voice (encoded digitally orotherwise), or other information. In a preferred embodiment, the B field209 may also comprise specified information for establishingcommunication links between base stations 104 and user stations 102. TheB field 209 may also comprise its own FCW or CRC code 211 having sixteenbits (with 160 bits of information, a total of 176 bits).

In a preferred embodiment, there may be 32 air channels is 203; themajor frame 201 may therefore comprise 32 minor frames 202 in sequence.Thus, each minor frame 202 may be about 307 microseconds long, each airchannel 203 (in a TDD or TDMA system) may be about 667 microsecondslong, and each major frame 201 may be about 20 milliseconds long. In apreferred embodiment, there may be 160 bits transmitted per air channel203; thus the 32-channel system would have about a 256 kilobits/secondtotal two-way data rate. Other time values are shown in the figure.

In a preferred embodiment, information may be transmitted at a rate offive bits each 6.4 microseconds, using a 32-ary code-shift keyingtechnique. Thus, each 6.4 microseconds, one of 32 different codes may betransmitted, with 32 different possibilities equalling five bits ofinformation. In an alternative preferred embodiment, one of 16 differentcodes may be transmitted, with an additional phase bit on the carrier(or, in a second alternative, more than one phase bit on the carrier),again with 32 different possibilities equalling five bits ofinformation.

In one embodiment, a minor frame 203 may operate in an asymmetric modein the sense that the greater portion of a minor frame 202 is devoted toeither the base transmission 204 or the user transmission 206. Highspeed data transport in either direction (i.e., from the base station104 to the user station 102, or vice versa) can be provided in theasymmetric mode, with or without acknowledgment and/or ARQ.

A particular sub-mode of the above described asymmetric mode may bereferred to as broadcast mode in which essentially the entire minorframe is devoted to one-way communication. In the broadcast mode, one ormore broadcast sub-channels may be identified by a special broadcastidentifier. Up to 255 broadcast channels may be so identified. For thesepoint-to-multipoint applications, broadcast frames are not acknowledged.

Control Pulse

A user station 102 in a cellular environment preferably has means forcontrolling transmission power to avoid interference with adjacentcells. Unlike a fixed station environment, in which antenna locations,patterns and fixed station transmission power may be adjusted forminimal interference with other fixed stations, the nature of a cellularenvironment with mobile user stations 102 is such that there can ariseconflict between user stations 102 at intersecting cell boundaries. Thiscreates the need for some power control in the user stations 102. Forexample, a user station 102 operating at the boundary of coverage of abase station 104 may need to transmit at full power to stay in contact.On the other hand, a user station 102 operating relatively close to itsown base station 104 may not need to transmit full power to have goodcontact. By proper power control, user stations 102 may maintainadequate contact with base stations 104 without unduly interfering withneighboring cell transmissions, allowing RF channel reuse in nearbycells. Power control may also reduce interference with fixed microwaveusers and conserve battery power in user stations 102 such as handheldunits.

The present invention achieves power control in one embodiment by use ofa power control pulse transmitted periodically from each user station102. After establishment of a communication link, described with regardto FIG. 3 herein, a control pulse time 213 and a third gap 214 may bereserved just prior to the start of the minor frame 202, in which theuser station 102 transmits a control pulse 215. The control pulse 215provides to the base station 104 a power measurement of the air channel203 indicative of the path transmission loss and link quality. Each userstation 102 generally transmits its control pulse 215 in the minor frame202 allocated to it (e.g., seized by the user station 102).

The control pulse 215 may be received by the base station 104 and usedby the base station 104 to determine information about the communicationlink it has with the user station 102. For example, the base station 104may determine, in response to the power, envelope, or phase of thecontrol pulse 215, the direction or distance of the user station 104,and the degree of noise or multipath error to which the communicationlink with the user station 102 may be prone.

In response to receiving the control pulse 215, the base station 104determines the quality of the received signal including, for example,the received power from the power control pulse 215 and thesignal-to-noise or interference ratio. The base station 104 then sends amessage to inform the user station 102 to adjust its power if needed.Based on the quality of the received signal, the base station 104 maycommand the user station 102 to change (increase or decrease) itstransmit power by some discrete amount (e.g, in minimum steps of 3 dB)relative to its current setting, until the quality of the control pulse215 received by the base station 104 is above an acceptable threshold.

Similarly, if the base station 104 knows the power setting of the userstation 102, then the base station 104 can adjust its own power as well.The base station 104 may adjust its power separately for each minorframe 202.

A preferred power control command pulse from the base station 104 to theuser station 102 may be encoded according to Table 5-1 below:

                  TABLE 5-1                                                       ______________________________________                                        Power Control Command                                                                           Adjustment                                                  ______________________________________                                        000               No change                                                   001               -3 dB                                                       010               -6 dB                                                       011               -9 dB                                                       100               +3 dB                                                       101               +6 dB                                                       110               +12 dB                                                      111               +21 dB                                                      ______________________________________                                    

Although preferred values are provided in Table 5-1, the number of powercontrol command steps and the differential in power adjustment betweensteps may vary depending upon the particular application and the systemspecifications.

While power control is thus desirable, a problem in some conventionalTDMA systems is that the length of the polling loop (e.g, the majorframe 201) is too long to allow the latest user transmission to be veryuseful for estimating the channel losses and impairments. In otherwords, the latency of the polling loop signals may prevent the use ofclosed loop power control. However, the described embodiment allows fora power control sequence that may be effectively carried out in arelatively short span of time, thereby allowing closed loop powercontrol. Preferably, the elapsed time encompassing transmission of thecontrol pulse 215, the base transmission 204, and the start of the usertransmission 206 is kept relatively short (e.g., less than 500 μsec orroughly 2.5% of the duration of the major frame 201), allowing systemresponse to be fast enough to counteract small scale multipath fadingeffects and propagation shadow effects.

The base station 104 may also use the control pulse 215 to measure thetime delay from a user station 102 and thereby estimate the distance ofthe user station 102 from the base station 104. For 911 support, a userstation 102 can provide control pulses 215 to multiple base stations 104for rough location estimation in emergency situations.

In a preferred embodiment, the base station 104 may have a plurality ofantennas for reception and transmission on the communication link withthe user station 102, and may select one of that plurality of antennasfor reception and/or transmission, in response to the determination thebase station 104 may make in response to the control pulse 215. The basestation 104 may make the determination of which antenna to use based onthe quality of the signal received from the control pulse 215transmitted by the user station 102. Because the base station can bothreceive and transmit on the antenna having the best received signalquality from the control pulse 215, the user stations 102 benefit fromantenna selection diversity even though they might not have explicitantenna diversity capabilities at the user station 102. The controlpulse 215 permits spatial diversity control to be updated during eachminor frame 202. Preferably, the base station 104 employs a high speedTDD technique such that the RF channel characteristics do not changewithin the time of the minor frame 202.

Information relating to the control pulse 215 for a particular userstation 102 may be transferred as information in control traffic fromone base station 104 to another base station 104 in the case of a basestation assisted handoff.

It should be noted that, in the preferred TDMA system described herein,the requirement of strict RF transmitter output power control is notnecessary to resolve the "near-far" problem commonly experienced in CDMAsystems. The purpose of the control pulse 215 is primarily to reducebattery consumption in user stations 102, to minimize interference oftransmissions among neighboring cells 103 which may be operating on thesame or adjacent RF channels, and to minimize interference with nearbyfixed microwave users.

The control pulse 215 may also serve as a synchronization preamble fordetermining the beginning of M-ary data symbols within the minor frame202. A power control command pulse, similar in length to the controlpulse 215, transmitted by the base station 104 during the basetransmission 204 or otherwise may likewise be used as a synchronizationpreamble at the user station 102, in addition to providing a powercontrol command to adjust the power output level at the user station102.

Base Station Output Power

Because a single base station 104 may communicate with a large number ofuser stations 102 (e.g., as many as 64 user stations 102) at a giventime, each of whose distance from the base station 104 may vary fromnear zero up to the radius of the cell 103, it may not be practical tocontrol the transmitter power of the base station 104 in order tomaintain a near-constant received power level at each user station 102during each minor frame 202. Output power control of the transmitter atthe base station 104 could require a large change (e.g., more than 40dB) in transmit power during each minor frame 202 (e.g., every 625 μs)of the major frame 201. As an alternative to providing power control ona minor frame 202 by minor frame 202 basis, output power control at thebase station 104 can be averaged over a longer time interval than eachminor frame 202.

Antenna Characteristics

In one aspect of the invention, the reciprocal nature of time divisionduplex (TDD) permits common antennas to be used for transmit and receivefunctions at both the base station 104 and the user stations 102,without the need for antenna diplexers. Common antennas can be used totransmit and receive because these functions are separated in time ateach of the terminals. Further, because TDD utilizes the same RFfrequency for the transmit and receive functions, the channelcharacteristics are essentially the same for both the base station 104and a particular user station 102.

The use of common antennas results in simplicity of the base station 104and user station 102 terminal designs. Further, use of the same RFfrequency and antenna for both transmit and receive functions at thebase station 104 and the user station 102 provides reciprocalpropagation paths between the base is station 104 and user station 102terminals. This reciprocal nature allows the base station 104 to use thechannel sounding of the control pulse 215 transmitted by the userstation 102 to determine the two-way path loss between the base station104 and the user station 102, and also to determine which of the spatialdiversity antennas at the base station 104 to use, both to receive fromthe user station 102 and to transmit to the user station 102.

Different types of antennas may be used by the base station 104,depending on the type of application. For low density suburban or ruralapplications an omnidirectional antenna may be used to provide maximumcoverage with the fewest base stations 104. For example, anomnidirectional antenna may be employed having a vertical gain ofapproximately 9 dB. The 9 dB of gain permits a relatively large radiuscell even with an omnidirectional horizontal pattern.

In suburban and low density urban areas, directional antennas with 120degree azimuth beamwidths and 9 dB vertical gain may be used at the basestation 104 so that a cell 103 can be sectorized into three parts, witheach sector accommodating a full load of user stations 102 (e.g., 32full duplex user stations 102).

The use of TDD also permits utilization of a single steered phased arrayantenna at the base station 104 for applications requiring a high gain,highly directional antenna. Similar deployment in CDMA or FDMA systemswould, in contrast, be more complex and costly, as they may requiresimultaneous steered beams for each user station 102 within the cell103.

For example, to permit a single base station 104 to cover large,sparsely populated area, a steered array antenna with up to 20 dB ofhorizontal directivity can be used. Such an antenna is sequentiallysteered to each user station 102 within a cell 103 at each minor frame202. The same antenna may be used for both transmission and reception,as noted, providing reciprocal forward and reverse link propagationcharacteristics. The steered array antenna may utilize circularpolarization so that high level delayed clutter signals reflected frombuildings or other obstructions within the beam path do not interferewith the received signals from the user stations 102. As reflectedsignals are typically reversed in polarization, they will be rejected bythe circularly polarized antenna. It should be noted that such highgain, directional antennas also reduce the delay spread in severemultipath environments by rejecting multipath components arriving fromoutside the main beam of the antenna.

In one embodiment, the user station 102 employs a halfwave dipoleantenna which is linearly polarized and provides a gain of 2 dB with anomnidirectional pattern perpendicular to the antenna axis. At a nominalfrequency of 1900 MHz, a half wavelength is approximately 3 inches,which fits well within a handset envelope.

Message Types And Protocol

FIG. 3 shows message types and a protocol which uses those messagetypes.

In a preferred embodiment, messages (base transmissions 204 and usertransmissions 206) may be one of three types: a general poll message301, a specific poll message 302, and an information message 303. When amessage is transmitted by a user station 102, it is called a "response",e.g., a general poll response 304, a specific poll response 305, and aninformation response 306.

User Station Initiation of a Link

A user station 102 may "acquire" a base station 104 by a sequence ofhandshaking steps. At a general poll step 307, the base station 104 maytransmit its general poll message 301 on an air channel 203 as part of aminor frame 202. The user station 102 receives the general poll message301 and, if and only if it was received without error, transmits itsgeneral poll response 304 on the same air channel 203. The general pollmessage 301 comprises a base ID 308, which may be 32 bits long, whichmay be recorded by the user station 102. In like manner, the generalpoll response 304 comprises a user ID 309, which may be 32 bits long,which may be recorded by the base station 104. The base ID 308 may beused during handoff, as noted herein.

Upon receiving a general poll response 304, at a specific poll step 310,the base station 104 may transmit a specific poll message 302,comprising the user ID 309 received by the base station 104 as part ofthe general poll response 304. The specific poll message 302 may betransmitted on the same air channel 203 as the general poll message 301,or may be transmitted on another air channel 203, so long as the userstation 102 is able to find it.

The user station 102 may monitor all air channels 203 for its specificuser ID 309. The user station 102 receives the specific poll message 302and, if and only if it was received without error and with the same userID 309, transmits its specific poll response 305 on the same air channel203. The specific poll response 305 comprises the same user ID 309 asthe general poll response 304.

In a preferred embodiment, however, the specific poll message 302 may beeliminated as redundant. The user station 102 may therefore follow thegeneral poll response 304 with a specific poll response 305 on aselected air channel 203. This air channel 203 may be designated by thebase station 104 in a part of the information field 209 of the generalpoll message 301, it may be designated by the user station 102 in a partof the information field 209 of the general poll response 304, or it maybe selected by the user station 102 in response to an unoccupied airchannel 203 (e.g., the user station 102 may seize an unoccupied airchannel 203). The latter of these three alternatives is presentlypreferred by the inventors.

Upon receiving a specific poll response 305 comprising a user ID 309which matches that of the general poll response 304, at alink-established step 311, the base station 104 may transmit aninformation message 303. At this point, the base station 104 and userstation 102 have established a communication link 312 on a designatedair channel 203, typically the air channel 203 originally polled by thebase station 104, but possibly a different air channel 203. The basestation 104 may couple a telephone line to that air channel 203, and theuser station 102 may begin normal operation on a telephone network(e.g., the user station 102 may receive a dial tone, dial a number, makea telephone connection, and perform other telephone operations). Thebase station 104 and user station 102 may exchange information messages303 and information responses 306, until the communication link 312 isvoluntarily terminated, until faulty communication prompts the userstation 102 to re-acquire the base station 104, or until handoff of theuser station 102 to another base station 104.

Should more than one user station 102 respond to a general poll message301 in the same minor frame 202, the base station 104 may advertentlyfail to respond. The lack of response from the base station 104 signalsthe involved user stations 102 to back off for a calculated timeinterval before attempting to acquire the same base station 104 usingthe general poll message 301 and general poll response 304 protocol. Theback-off time may be based upon the user ID 309, and therefore each userstation 102 will back off for a different length of time to preventfuture collisions.

In one embodiment, the general poll message is sent by a base station104 on one or more currently unoccupied air channels 203. Originally, atpower-up of the base station 104, the base transmission 204 for all ofthe air channels 203 may therefore contain the general poll message 301.

Base Station Initiation of a Link

When an incoming telephone call is received at a base station 104, at anincoming-call step 313, the base station 104 transmits a specific pollmessage 302 with the user ID 309 of the indicated recipient user station102 (skipping the general poll message 301 and the general poll response304) on an available air channel 203.

Each user station 102 listens for the specific poll message 302repeatedly on each air channel 203 so as to receive the specific pollmessage 302 within a predetermined time after it is transmitted. Thuseach user station 102 may periodically receive each air channel 203 insequence so as to listen for the specific poll message 302.

When the specific poll message 302 is received, the user station 102compares the user ID 309 in the message with its own user ID, and ifthey match, continues with the link-established step 311. The basestation 104 may thus establish a communication link 312 with any userstation 102 within communication range.

Link Expansion and Reduction

The data transmission rate between a base station 104 and a user station102 may be expanded or contracted over the duration of the communicationlink.

In one embodiment, the base station 104 increases the data transmissionrate by transmitting multiple information messages 303 to the userstation 102 during a major frame 201, essentially allocating multipleminor frames 202 to a single user station 102. These higher data rates,also known as "super rates", are implemented by means of a targetedinformation message 303. In a targeted information message 303, the basestation 104 may transmit the user nickname 212 in the D field 208, alongwith information to be transmitted to the designated user station 102 inthe B field 209. When the user station 102 detects the user nickname 212assigned to it, it receives the targeted information message 303.

In a preferred embodiment, the user nickname 212 may be transmitted bythe base station 104 to the user station 102 in the specific pollmessage 302. In an embodiment where the specific poll message 302 hasbeen eliminated as redundant, the user nickname 212 may be transmittedby the base station 104 to the user station 102 bit-serially in adesignated bit of the header field 207.

Because the data transmission rate is related to the number of minorframes 202 allocated to a specific user station 102, the datatransmission rate increases in steps of, for example, 8 Kbps. It iscontemplated that up to the full bandwidth of the base station 104--thatis, up to all 32 full duplex slots or 256 Kbps (full duplex)--may beassigned to a single user station 102.

The invention also provides in another aspect data rates lower than thebasic rate (i.e., less than one minor frame 202 per major frame 201 orless than 8 Kbps). The lower data rate is accomplished by skipping majorframes 201 on a periodic basis. Thus, data rates such as 4 Kbps, 2 Kbps,and so on can be provided. In one embodiment, up to 24 consecutive majorframes 201 may be skipped, providing a minimum data rate of 320 bpsefficiently (i.e., without using rate adaptation). Intermediate rates oreven lower rates may be obtained by using rate adaptation.

The capability of providing variable data rates on demand, includingavailability of an asymmetric mode in a given minor frame 202 describedearlier, provides an efficient and is flexible data conduit for a widearray of data, video, multi-media and broadcast applications. Forexample, each minor frame 202 can be configured with the majority of theminor frame 202 duration allocated to either the base transmission 204or the user transmission 206, or can be configured with a symmetric 20distribution in which half of the minor frame 202 duration is allocatedto both the base transmission 204 and the user transmission 206.Typically, voice traffic utilizes a symmetric distribution as either endof the link may send voice traffic.

In a data exchange, however, more data is typically sent in onedirection and less in the other. For instance, if fax data is being sentto a user station 102, then a higher data rate for the base transmission204 would be advantageous and is supportable with the describedconfiguration. For even higher data rate applications, a particular basestation 104 or user station 102 may be assigned multiple minor frames202 within a single major frame 201. These high data rate modes cansupport, for example, enhanced voice quality, video data or broadcastdata applications.

Handoff and Network Maintenance

Once a base station 104 and user station 102 have established acommunication link 312, during the link-established step 311 the userstation 102 may receive all information messages 303 and transmit allinformation responses 306 on the same air channel 203 or on specifiedmultiple air channels 203. This arrangement leaves the remainder of themajor frame 201 free for other activities. In a preferred embodiment,one such activity is to interrogate other base stations 104 and maintainnetwork information such as link quality and channel availability atnearby base stations 104 in order to facilitate handoffs from one basestation 104 to another base station 104.

In a preferred embodiment, base stations 104 transmit networkinformation as part of the general poll message 301 and the specificpoll message 302, in a channel utilization field 314 or otherwise. Thenetwork information may include, for example, the identity of nearbybase stations, the identity or relative amount of free channels at aparticular nearby base stations and/or at the current base station, linkquality for nearby base stations and/or the current base station, andfrequencies and spread spectrum code sets used by the nearby basestations.

At a network-maintenance step 315, the user station 102 may listen onone or more different air channels 203, other than the one(s) currentlybeing used by the user station 102, for the general poll message 301 andthe specific poll message 302 from nearby base stations 104. The userstation 102 continues to communicate on its designated air channel(s)203 with its current base station 104 and responds as necessary toinformation messages 303 from that base station 104. However, unless ahandoff procedure is initiated as described below, the user station 102does not transmit in response to other nearby base stations 104 andtherefore does not occupy air channels 203 of those base stations 104.

It is contemplated that the system may perform either a "make beforebreak" handoff for seamless, undetectable handoffs, or a "break beforemake" handoff in emergency situations where all communications with abase station 104 are lost prior to a new connection being established.

In a "make before break" handoff, if the communication link 312 betweenthe base station 104 and the user station 102 is too faulty, then theuser station 102 may acquire one of the nearby base stations 104 in likemanner as it acquired its current base station 104. Such a handoffprocedure may be further explained with reference to FIG. 4.

In FIG. 4, it is assumed that a user station 102 presently incommunication with a current or original base station 405 has determinedit to be desirable to transfer communication to a different base station104, such as a first terminal base station 410 coupled to a common basestation controller 407, or a second terminal base station 406 coupled toa different base station controller 408. A handoff to the first terminalbase station 410 will be termed an "intra-cluster" handoff, while ahandoff to the second terminal base station 406 will be termed an"inter-cluster" handoff. The following explanation will focus on anintra-cluster handoff to the first terminal base station 410, but manyof the steps are the same as with an inter-cluster handoff, and thesalient differences between an intra-cluster and inter-cluster handoffwill be noted as necessary.

In general, when the user station 102 determines that a handoff isappropriate, the user station 102 acquires an air channel on the new orterminal base station 410 and notifies the base station controller 407coupled to the current base station 405 to switch the incoming phoneline from the current base station 405 to the new base station 410.

More specifically, a handoff procedure may be initiated when thereceived signal level at a user station 102 falls below an acceptablelevel. While the user station 102 receives bearer traffic from itsoriginating base station 405, the user station 102 measures the receivedsignal quality (e.g., RSSI) of its communication link 312. The receivedsignal quality value, together with measurements of the current frameerror rate and type of errors, determines the overall link quality. Ifthe overall link quality drops below a first threshold (the measurementthreshold), the user station 102 begins searching for available airchannels 203 (i.e., time slots), first from the originating base station104, and then (using appropriate frequencies and spread spectrum codes)from neighboring base stations 104 of adjacent or nearby cells 103. Theuser station 102, as mentioned, preferably has obtained informationregarding the identities of neighboring base stations 104 (includingspread spectrum code set and frequency information) from the originatingbase station 405 by downloading the information to the user station 102during traffic mode or otherwise.

As the user station 102 scans potential new air channels 203 using theappropriate frequency and/or spread spectrum code set, the user station102 measures and records the received signal quality. The user station102 reads a field carried in all base transmissions 204 which describesthe current time slot utilization of the base station 104. The userstation 102 uses these two pieces of information to form a figure ofmerit for the new base station signals, including the originating basestation 405, and then sorts the base stations 104 by figure of merit.This procedure allows the user station 102 to evaluate the quality ofavailable air channels 203 for both the originating base station 405 andother nearby base stations 104.

If an air channel 203 (or air channels 203, as the case may be) for theoriginating base station 405 has better quality than that of any basestation 104 in adjacent or nearby cells 103, a time slot interchange(TSI) handoff is considered, which maintains the link to the originatingbase station 405 on a different air channel 203 than was previouslybeing used by the user station 102.

If the link quality drops below a second threshold level, then the userstation 102 (during a no-bearer time slot) requests a handoff from thebase station 104 with the highest figure of merit (which could be a TSIhandoff with the originating base station 405). The handoff is requestedby seizing an air channel 203, sending a handoff message request, andwaiting for an acknowledgment from the new base station 410. The handoffsignaling message contains a description of the circuit connecting theoriginating base station 405 to the network, which description waspassed to the user station 102 at call establishment time. If the newbase station 104 accepts the handoff request (by acknowledging), thenthe new base station 104 becomes the terminal base station 410. Notethat the user station 102 maintains its original air channel 203connection with the originating base station 405 during this handoffprocedure, at least until a new air channel 203 is acquired.

To complete an intra-cluster handoff, at a handoff step 316 the userstation 102 transmits to the new base station 410 the base ID 308 of theold base station 405. The old base station 405 and new base station 410may then transfer the handling of any telephone call in progress.

More specifically, the terminal base station 410 sends a message in theform of a "note" (as previously described) to its base stationcontroller 407, requesting that the original circuit be switched fromthe originating base station 405 to the terminal base station 410. Ifthe base station controller 407 is common to both the originating basestation 405 and terminal base station 410, the handoff is termed anintra-cluster event, and the base station controller 407 bridges thecircuit from the originating base station 405 to the terminal basestation 410. The base station controller 407 then sends acircuit-switchcomplete note to the originating base station 405 and alsoto the terminating base station 410, commanding the latter to continuethe handoff process.

In the case of an inter-cluster handoff, the base station controller 408is not common to both the originating base stations 104 and the terminalbase station 406. For these types of handoffs, as with intra-clusterhandoffs, the terminal base station 406 sends a message in the form of anote to its base station controller 408, requesting that the originalcircuit be switched from the originating base station 405 to theterminal base station 406. The base station controller 408 translatesthe handoff note into the signaling language of the network host 409(e.g, a PCSC) and requests an inter-cluster handoff at the networklevel.

In some network architectures, the host network 409 cannot accept ahandoff request from a terminating base station controller 408, in whichcase an intermediate step is taken. The handoff request may be sent viaan X.25 link to the base station controller 407 connected to theoriginating base station 405. The originating base station controller407 then translates the handoff request and relays it to the networkhost 409. The network host 409 acknowledges the circuit switch to theoriginating base station controller 407, which then sends acircuit-switch-complete note to the terminal base station 406.

When the terminal base station 406 receives the circuit-switch-completenote, the terminal base station 406 begins paging the user station 102with a specific poll, and the originating base station 405 signals theuser station 102 to transfer to the terminal base station 406. When theuser station 102 receives the signal to transfer to the terminal basestation 406, or if the link is lost during the handoff process, the userstation 102 switches to the terminal base station 406 and searches for aspecific poll message 302. When the user station 102 receives thespecific poll message 302, the user station 102 completes the connectionto the terminal base station 406, and the handoff procedure is finished.

Should the link between the user station 102 and the originating basestation 405 or terminating base station 406 (or 410) be completelybroken at any time, the user station 102 will search for the highestquality base station 104 on its list of potential handoffs, and attempta handoff without communication with its previous base station 405. Thiscapability allows the user station 102 to recover from situations inwhich the original link was broken before the normal handoff procedurecould be completed.

An intra-cluster handoff, including re-establishment of bearer channeltraffic, may ordinarily take from less than 10 milliseconds to as muchas 40 milliseconds. Since under normal circumstances the handoff time isless than one polling loop interval, bearer packets will continue to theuser station 102 with no interruption. Inter-cluster handoff times arepartially dependent upon the delays inherent in the host network 409 andare not always easily predictable.

A unique aspect of the above described "mobile directed" or "mobilecentric" handoff technique is that the user station 102 makes thedecision to handoff between cells and directs the base stationcontroller or network to make a line switch once an alternative basestation 104 is acquired. This approach is quite different from a"network directed" or "network centric" approach such as used in systemssuch as AMPS, IS-54 cellular, and GSM. The mobile centric approach alsodiffers significantly from so-called "Mobile Assisted Handoff" (MAHO) inwhich the network collects information and directs all or most of thehandoff functions, thereby utilizing the user station 102 primarily asan additional listening post with the network still directing thehandoff. The MAHO technique therefore ordinarily requires significantsignaling and messaging between base stations, base station controllers,and switches, causing handoffs to take much longer than with the mobilecentric techniques described herein.

A major benefit of the mobile centric approach is that it may allow formobile speed handoffs (e.g., 65 MPH) even in very small or very largecells, such as cells ranging from as small as under 1000 feet to aslarge as 20 miles in diameter.

The system is also capable of performing a "break before make" type ofhandoff as well. A "break before make" handoff is typified in asituation where sudden shadowing occurs, such as when a connection withthe current base station 405 is lost due to a severe signal blockage(e.g. worse than 40 dB) near the limit of the cell range such as canoccur when turning a corner quickly in a dense urban high rise area. Insuch a situation, the user station 102 checks its previously created"priority list" of available base stations in the vicinity and attemptsto establish contact with a new base station 104, perhaps on a newfrequency and/or a new time slot. The user station 102 may include aspart of its control logic a "persistence" parameter which will precludecall tear down from occurring before a duplex connection is fullyreestablished.

The true "hard handoff" problem (i.e., a lost air channel) may in manyinstances be handled very quickly through the ability of the userstation 102 to re-acquire the original base station 405 or to acquire adifferent base station 104 very rapidly even when no information isavailable to the user station 102 when the link was lost. Even in suchan emergency "break before make" handoff situation, the handoff mayordinarily be accomplished in as little as 16 to 250 milliseconds. Incontrast, complete loss of a link in traditional cellular architecturesbecomes a "dropped call."

One problem that may occur during handoff is a situation in which thereare repeated attempts to switch between two or more base stations 104during times, for example, when the measured quality of the receivedsignals from two competing base stations 104 is very close, or whenenvironmental effects cause rapidly changing deviations in the relativemeasured signal quality of the signals from competing base stations 104.The repeated switching between competing base stations 104 may bereferred to as "thrashing" and may have the undesirable effect ofconsuming excess capacity from the network. In order to reduce theeffect of thrashing, hysteresis measurements from multiple base stations104 may be maintained by the user station 102 so that a handoff does notoccur until the quality of the signal from a new base station 104exceeds the quality of the signal of the original base station 405 by apredetermined margin. In such a manner, important air channel resourcesin the network may be preserved.

In rare instances, two user stations 102 on the same minor frame 202 indifferent cells 103 but on the same frequency may encounter propagationcharacteristics in which the spatial and code separation areinsufficient to prevent bit errors, thus causing the user stations 102to begin experiencing degradation of their RF links. In such cases, atime slot interchange (TSI) may be performed wherein one or both of theconflicting user stations 102 are assigned different minor frames 202within their respective major frames 201 to eliminate furthercollisions. Such a procedure may be viewed as the time domain equivalentof dynamic channel allocation as the system either assigns an unoccupiedair channel 203 to the user station 102 or switches the user station's102 minor frame 202 with that of another user station 102 in the samecell 103 which is geographically removed from the interference.

Security and Error Handling

The protocol of the invention protects communications against errors inseveral ways: protocol handshaking, user ID verification andreverification, and synchronization by reacquiring the base station.Handshaking, verification and synchronization protect both the basestation 104 and the user station 102 from receiving telephone calls inprogress on any other air channels 203.

Handshaking provided by the general poll step 307 and the specific pollstep 310 requires that the proper message having the proper header betransmitted and received, and in the proper sequence. In each message,the header field 207 (sixteen bits) is protected by a CRC code 211 (fourbits); an error in the header field 207 or in the CRC code 211 indicatesan error and will cause the protocol to restart handshaking with thegeneral poll step 307.

The user ID is verified twice, once by the base station 104 and once bythe user station 102. In the general poll message 301 and specific pollmessage 302, the user ID 309 is protected by a CRC code 211 (sixteenbits), in like manner as the CRC code 211 for the header field 207. Anerror in the user ID 309 or in the CRC code 211 will cause the protocolto restart handshaking with the general poll step 307.

At the link-established step 311, the base station 104 and the userstation 102 are protected against drift and/or desynchronization, evenwhen transmission or reception are interrupted. When a threshold for anerror rate is exceeded, the base station 104 and user station 102 eachindependently stop sending data in information messages 303 andinformation responses 306, and return to the specific poll step 310 forresynchronization. In an embodiment where the specific poll message hasbeen eliminated as redundant, the base station 104 and the user station102 may determine resynchronization by means of a designated bit in theheader field 207.

At the specific poll step 310, the base station 104 transmits thespecific poll message 302 and the user station 102 searches the majorframe 201 for a specific poll message 302 having a user ID 309 whichmatches its own user ID 309. After this handshaking succeeds, the basestation 104 and user station 102 return to the link-established step 311and continue transmitting and receiving information messages 303 andinformation responses 306.

This technique for recovery from desynchronization, also called"reacquiring the base station," has the advantage that both the basestation 104 and the user station 102 independently reverify the user ID309 before communication is resumed. This assures that the base station104 and the user station 102 stay in synchrony and communicate only onthe agreed air channel 203. Should the base station 104 and the userstation 102 be unable to reestablish the communication link 312, thetelephone call will be terminated by the base station 104.

At the link-established step 311, the base station 104 also repeatedlyand periodically transmits the user ID 309 in the D field 208 of theinformation message 303. The user station 102 checks the user ID 309 toassure that the base station 104 and the user station 102 are eachcommunicating on the proper air channel 203. If this user ID 309 doesnot match, it returns to the specific poll step 310 to reacquire thebase station 104, as noted above.

Protocol Flexibility

The protocol described above provides flexibility with a small number ofunique messages. The protocol is immune to changes in polling looplength and in the number of air channels allowed. The number ofsimultaneous users is therefore responsive to voice compression and datarate constraints and not by the protocol. The protocol also provides foran unlimited number of user stations in a given area, with the provisionthat the number of simultaneous calls cannot exceed the number of airchannels. An unlimited number of base stations are also supported,making base station geography a function of available frequencies andrange, not of protocol. The ability to interrogate and acquire alternatebase stations in the presence of faulty communication provides for theexpansion of a microcell network which may use base station handoff toroute calls to base stations within range.

System Synchronization

In order to maximize system throughput capacity, the TDMA frame timesfor all base stations 104 within a geographical region are preferablysynchronized to within a specified tolerance. For example, in oneembodiment, all base stations 104 begin transmissions for the same framewithin 6 microseconds.

The primary data timing standard in a digital network backhaul system,such as Ti, ISDN BRI, or PRI, is the public switched telephone network(PSTN) timing standard. To prevent data precession into over run orunder run, all base station controllers 105 and base stations 104 insuch systems are synchronized to the PSTN timing standard.

At the system level, a GPS receiver is used at each base stationcontroller 105 (and optionally at each base station 104) to generate theprimary reference timing marker for the TDMA frame timing. This markeris captured at the base station controller 105 every second andtransmitted to the attached base stations 104. A base station controllermay temporarily turn off any major frame 201 or minor frame 202 of agiven cell 103 which may be interfering with a neighboring cell 103.

Each base station 104 provides the basic TDMA loop timing structure forits cell or sector. As previously noted, a synchronization preamble inthe form a control pulse 215 or power control command is transmitted atthe beginning of each minor frame 202 by the user station 102 and thebase station 104, respectively. When the appropriate preamble,consisting of a code sequence 48 chips in length, is received, a digitalcorrelator (i.e., a matched filter) attuned to the specific preamblegenerates an internal synchronization pulse which may be very brief(e.g., two chips in duration, or 400 nanoseconds). The internalsynchronization pulse may then be used to synchronize the start of M-arysymbol detection process.

Alternative Embodiments

While preferred embodiments are disclosed herein, many variations arepossible which remain within the concept and scope of the invention, andthese variations would become clear to one of ordinary skill in the artafter perusal of the specification, drawings and claims herein.

For example, information which is transmitted from transmitter toreceiver is referred to herein as "data", but it would be clear to thoseof ordinary skill in the art, after perusal of this application, thatthese data could comprise data, voice (encoded digitally or otherwise)error-correcting codes, control information, or other signals, and thatthis would be within the scope and spirit of the invention.

Moreover, while the specification has been described with reference toTDMA multiplexing of air channels, it would be clear to those ofordinary skill in the art, after perusal of this application, that airchannels may be multiplexed by other means, including FDMA (frequencydivision multiplexing), by assigning air channels to differing frequencybands, CDMA (code division multiplexing), by assigning air channels todiffering spread-spectrum spreading codes, other multiplexingtechniques, or combinations of these multiplexing techniques, and thatthis would be within the scope and spirit of the invention.

What is claimed is:
 1. A multi-mode time division multiple accesscommunication system, comprising:a major time frame, said major timeframe repeating periodically, a plurality of minor time frames in saidmajor time frame, each of said minor time frames comprising a basestation transmission allocation and a user station transmissionallocation, said base station transmission allocation and said userstation transmission allocation collectively comprising more than halfof a minor time frame, a base station including a base stationtransmitter operating at a data bit transmission rate, a plurality ofuser stations each including a user station transmitter operating atsaid data bit transmission rate, and each of which communicates in atleast one of said minor time frames, wherein communication in each ofsaid minor time frames is carried out in one of a plurality of modesselected independently for each minor time frame, said plurality ofmodes comprising at least a first mode, a second mode, and a third mode,and wherein communication according to said first mode is characterizedby a first fixed base station transmission allocation equal to a firstfixed user station transmission allocation for each periodic repetitionof said major time frame until completion of a call, whereincommunication according to said second mode is characterized by a secondfixed base station transmission allocation not equal to a second fixeduser station transmission allocation for each periodic repetition ofsaid major time frame until completion of a call, and whereincommunication according to said third mode is characterized by a singledirection link transmission with no reverse link transmission.
 2. Themulti-mode time division multiple access communication system of claim 1wherein said first mode comprises a symmetric mode, said second modecomprises an asymmetric mode, and said third mode comprises a broadcastmode.
 3. The multi-mode time division multiple access communicationsystem of claim 1 wherein data communicated in said first mode comprisesvoice traffic.
 4. The multi-mode time division multiple accesscommunication system of claim 1 wherein said second user stationtransmission allocation exceeds said second base station transmissionallocation.
 5. The multi-mode time division multiple accesscommunication system of claim 1 wherein said second user stationtransmission allocation is less than said second base stationtransmission allocation.
 6. The multi-mode time division multiple accesscommunication system of claim 1 wherein data communicated in said secondmode comprises enhanced voice data, video data, or fax data.
 7. Amulti-mode time division multiple access communication system, wherein amajor time frame is divided into a plurality of minor time frames, saidsystem comprising:a base station including a base station transmitteroperating at a data bit transmission rate, said base stationcommunicating in at least one fixed-length base station transmissiontime period within a minor time frame, a plurality of user stations eachincluding a user station transmitter operating at said data bittransmission rate, and each of which communicates in at least onefixed-length user station time period within a minor time frame, saidbase station transmission time period and said user station time periodcollectively comprising more than half of a minor time frame, and aplurality of modes by which the relative lengths of said base stationtransmission time period and said user station transmission time periodare allocated on a continuing basis over a plurality of consecutivemajor time frames, said plurality of modes comprising a symmetric modein which said user station transmission time period is equal in lengthto said base station transmission time period for each of said pluralityof consecutive major time frames, an asymmetric mode in which said userstation transmission time period is unequal in length to said basestation transmission time period for each of said plurality ofconsecutive major time frames, and a broadcast mode in which no userstation transmission time period is designated for each of saidplurality of consecutive major time frames.
 8. The multi-mode timedivision multiple access communication system of claim 7 wherein saidbroadcast mode is characterized by a base station transmission without areverse link user station transmission, said base station transmissionbeing intended for receipt by a plurality of said user stations.
 9. Themulti-mode time division multiple access communication system of claim 7wherein said user station transmission time period exceeds said basestation transmission time period in said asymmetric mode.
 10. Themulti-mode time division multiple access communication system of claim 7wherein said user station transmission time period is less than saidbase station transmission time period in said asymmetric mode.
 11. Themulti-mode time division multiple access communication system of claim 7wherein data communicated in said symmetric mode comprises voicetraffic.
 12. The multi-mode time division multiple access communicationsystem of claim 7 wherein data communicated in said asymmetric modecomprises enhanced voice data, video data, or fax data.
 13. A methodcomprising the steps of:establishing a series of major time frames, eachmajor time frame divided into a plurality of minor time frames,selecting for each minor time frame used for communication a mode fromamong a plurality of modes for subsequent communication, said pluralityof modes comprising at least a symmetric mode and an asymmetric mode,dividing each minor time frame for which said symmetric mode is selectedinto two substantially equal fixed portions for duplex bearer trafficcommunication between a base station and a user station to which theminor time frame is assigned, and dividing each minor time frame forwhich said asymmetric mode is selected into two unequal fixed portionsfor duplex bearer traffic communication between said base station and auser station to which the minor time frame is assigned, one of said twounequal fixed portions having a first predefined duration regardless ofvoice activity and the other of said two unequal fixed portions having asecond predefined duration regardless of voice activity.
 14. The methodof claim 13 further comprising the step of communicating between saidbase station and each user station using spread spectrum communication.15. The method of claim 13 wherein said plurality of modes furthercomprises a broadcast mode, and further comprising the step oftransmitting a broadcast transmission from said base station in eachminor time frame for which said broadcast mode is selected withoutreceiving a corresponding reverse link transmission at said basestation.
 16. A time division multiple access communication systemcomprising:a repeating series of major time frames, each major timedivided into a plurality of minor time frames of substantially the samelength, and a base station capable of communicating with a plurality ofuser stations, said base station operating in one of a plurality ofdifferent modes in each minor time frame, one of said plurality ofdifferent modes being assigned to each minor time frame to governongoing communication within that minor time frame regardless of voiceactivity, said different modes including a first mode wherein two fixedequal portions of a minor time frame are allocated to base stationbearer traffic messages and user station bearer traffic messages, asecond mode wherein two fixed unequal portions of a minor frame areallocated to base station bearer traffic messages and user stationbearer traffic messages, one said two fixed unequal portions being of afirst predefined duration and the other of said two fixed unequalportions being of a second predefined duration, and a third mode whereinthe entirety of the time frame is allocated to said base station. 17.The time division multiple access communication system of claim 16wherein said base station transmits and receives messages using spreadspectrum communication.
 18. The time division multiple accesscommunication system of claim 16 wherein said base station and said userstations transmit data at the same bit rate.
 19. The multi-mode timedivision multiple access communication system of claim 1 wherein saidbase station transmission allocation and said user station transmissionallocation are adjacent in time.
 20. The multi-mode time divisionmultiple access communication system of claim 1 wherein said basestation transmitter and said user station transmitter transmit over thesame frequency band.
 21. The multi-mode time division multiple accesscommunication system of claim 7 wherein said base station transmissiontime period and said user station transmission time period aretime-adjacent.
 22. The multi-mode time division multiple accesscommunication system of claim 7 wherein said base station transmitterand said user station transmitter transmit over the same frequency band.23. The multi-mode time division multiple access communication system ofclaim 1 wherein, when in said third mode, said single direction linktransmission substantially comprises traffic messages.
 24. Themulti-mode time division multiple access communication system of claim 7wherein, when in said broadcast mode, messages transmitted during saidbase station transmission time period include non-signaling trafficmessages.
 25. The method of claim 15 wherein said step of transmitting abroadcast transmission from said base station in each minor time framefor which said broadcast mode is selected comprises the step oftransmitting at least some traffic messages during at least one minortime frame for which said broadcast mode is selected.
 26. A method ofcommunication, comprising the steps of:establishing a major time frame,said major time frame repeating periodically; establishing a pluralityof minor time frames in said major time frame for communication betweena base station and a plurality of user stations; allocating, on demand,minor time frames to user stations, such that each user stationcommunicating with said base station communicates in the same minor timeframe with each periodic repetition of said major time frame; selecting,for each minor time frame allocated to a user station, a communicationmode from among a plurality of communication modes, said plurality ofcommunication modes including at least a symmetric mode and anasymmetric mode; allocating, for each minor time frame for which saidsymmetric mode is selected, a first user-to-base transmission timeperiod and a first base-to-user transmission time period for allsubsequent communication, said first user-to-base transmission timeperiod and said first base-to-user transmission time period being ofsubstantially equal duration; allocating, for each minor time frame forwhich said asymmetric mode is selected, a second user-to-basetransmission time period and a second base-to-user transmission timeperiod for all subsequent communication, said second user-to-basetransmission time period and said second base-to-user transmission timeperiod being unequal in duration and having the same fixed length ateach occurrence of the minor time frame; for each minor time frame forwhich said symmetric mode is selected, at each periodic repetition ofsaid major time frame and regardless of voice activity detection,transmitting user-to-base messages from the user station to which theminor time frame is allocated to said base station only during saidfirst user-to-base transmission time period, and transmittingbase-to-user messages from said base station to the user station towhich the minor frame is allocated only during said first base-to-usertransmission time period; and for each minor time frame for which saidasymmetric mode is selected, at each periodic repetition of said majortime frame and regardless of voice activity detection, transmittinguser-to-base messages from the user station to which the minor timeframe is allocated to said base station only during said seconduser-to-base transmission time period, and transmitting base-to-usermessages from said base station to the user station to which the minorframe is allocated only during said second base-to-user transmissiontime period.